1. Field of the Invention
The present invention relates to an apparatus and method operable to embed a mass of additional information into quantized frequency image data.
2. Description of the Related Art
In recent years, digital content such as digitized audio and video data has been on the increase. Digital content provides a perfectly faithful reproduction of its original, and therefore it is an important issue to protect the copyright of the digital content. Illegally reproduced or distributed content cannot be differentiated from corresponding original content, and it is difficult to demonstrate evidence to assert the copyright of the original content. Accordingly, efforts have been made to protect the copyright of the digital content.
To this end, a typical “digital watermark” is used as one of the efforts. The digital watermark is a known art in which data is embedded into the audio and video data without allowing human beings to perceive the presence of the embedded data therein, and to extract the embedded data from the embedded data-containing audio and video data. According to the known art, copyright information such as on a copyright holder's name and copyright date is embedded as the digital watermark into the audio or video data in order to protect the copyright of the digital content. As a result, the embedded copyright information is detectable from illegally reproduced content, and the copyright holder of the content is identified to prevent the illegal reproduction of the digital content. In addition, digital content having the copyright information extensively embedded therein throughout the digital content makes it feasible to pinpoint specific locations of tampered data as well as the presence of the tampered data.
Several methods have been proposed to embed additional information into encoded image data having images compressed in accordance with any international standard specification such as JPEG and MPEG, and to extract the additional information from the encoded image data upon the decoding of the encoded image data.
For example, Patent Reference No. 1 (Published Japanese patent application 2002-330279) discloses a method operable to both embed and extract data from images.
To embed the data into the images, each original image is divided into several blocks, each of which is of 8×8 pixels. Subsequently, all of the blocks that form the entire image experiences discrete cosine transformation (DCT), thereby generating frequency image data or rather DCT coefficients herein. Each of the blocks is quantized using a quantization table, thereby providing quantized frequency image data in which a single coefficient at the highest frequency domain is replaced by one-bit data. The replacement allows the data to be embedded, as expected, into the quantized frequency image data. Thereafter, run-length encoding and Huffman coding are executed in order. Subsequently, a value corresponding to a single coefficient at the highest frequency domain among values in the quantization table is replaced by “1” (one).
In JPEG-encoding based apparatuses such as digital cameras and color facsimiles, input images are both encoded and compressed by hardware, and a considerable amount of cost is incurred when a digital watermark-embedding step is added to be followed by such an image-compressing and -encoding step. To avoid such an inconvenience, digital watermark information may be embedded into either encoded data or quantized frequency image data, after the receipt of either the encoded data or quantized frequency image data, in accordance with either a table used to encode such input data or a table used to quantize the input data.
However, according to Patent Reference No. 1, in order to embed massive additional information of at least several hundred bytes into the encoded image data having images compressed therein, the quantized frequency image data in each of the blocks must be replaced by one-bit data in accordance with quantities (in general, any number except for “1”) corresponding to desired pieces of data to be embedded.
DCT-coefficients in high frequency domain components often result in “0” (zero) because of the quantization, and a series of “0's” (zeros) sometimes appears extending to the tail of each of the high frequency domain components. Such a property is utilized by encoding-decoding processes. More specifically, the series of “0's” extending to the tail of the high frequency domain component is replaced by a particular code or an EOB (End of Block) in a zigzag-scanning path that begins at a direct-current component, and that terminates at the end of the high frequency domain component. The EOB-based replacement allows a chain of several “0's” up to several tens of “0's” to be replaced only by a single code, and codes in a considerably reduced amount are usable.
However, according to the known arts, when a single coefficient at the highest frequency domain is replaced by “1”, then the tail of the high frequency domain component results in “1”. This means that the chain of “0's” extending to the tail of the high frequency domain component is absent. As a result, the EOB-based replacement is unachievable, and codes are significantly increased in amount.
Although it cannot be impossible to replace DCT-coefficients in intermediate and low frequency domain components, it is to be noted that the replacement of a single coefficient at the highest frequency domain in accordance with the aforesaid art is not believed to result in much degradation in image quality. In contrast, the replacement of the DCT-coefficients in the intermediate and low frequency domain components inevitably degrades the image quality because the replaced DCT-coefficients are closely related to the image quality, and because they are relatively great in value.
Patent Reference No. 2 (Published Japanese patent application 2000-151973) discloses the embedding of a mass of additional information into image data. It is to be noted that a sum of absolute values of differences between frequency image data before information embedment therein and frequency image data after the information embedment therein is preferably as small as possible because an increased sum of the absolute values means a big change in image quality, or more specifically, means that post-embedded images having information embedded therein are considerably reduced in image quality than pre-embedded images.
Inverse quantization primarily multiplies each value in the quantization table by corresponding value of the quantized frequency image data. In general, the values in the quantization table considerably increase with a relatively high compression ratio. Since the quantization table provides very high values (e.g., “100”) as a result of the relatively high compression ratio, a slight change (e.g., “+1”) in value of the quantized frequency image data in accordance with Patent Reference No. 2 to embed the information thereinto brings about an increase (difference: +1*100=+100) several times up to several hundred or more as great as the slight change. This causes a problem in that the information embedment into the quantized frequency image data materially degrades decoded images in image quality.